1. Field of the Invention
This invention relates to techniques for controlling the attitude of a spacecraft and more particularly, to controlling spacecraft attitude with minimal vibration. The invention has particular application to spacecraft having large solar arrays in geosynchronous orbit wherein a solar array is continuously tracking the sun, as well as any apparatus where it is desirable to minimize vibration introduced by stepped excitation.
A class of spacecraft known as three-axis stabilized spacecraft employ a solar array to generate power for the spacecraft. The solar array must be maintained in a position normal to the sun to absorb the optimum amount of radiation. Because the solar array must be maintained normal to the sun, a servo controlled stepping mechanism, e.g., a stepping motor in connection with an appropriate gear train, is typically employed to cause the solar array to track the sun while the spacecraft is in constant relative rotation to the sun in an orbit about the earth. Other types of attitude control mechanisms, such as dc motors, prove to be relatively difficult to control and heavy in applications of this class. However, in theory, servo controlled dc motors would not generate oscillation. It is desirable to use stepper motors because stepper motors are relatively simple to control, reliable, lightweight and well adapted to continuous use.
One of the major problems with the use of the stepping motor is that the stepping action can excite a highly flexible array such that oscillation is induced within the spacecraft. The induced oscillation is particularly critical in spacecraft where absolute platform stability is desirable or required, such as platforms for high resolution optical imaging systems. Vibrations can cause deterioration of any inertia-sensitive operations of a spacecraft. Therefore, it is desirable to solve the problem of induced oscillation caused by a stepper motor.
Oscillation and jitter do not present a problem in certain classes of spacecraft where on-board systems are not sensitive to the oscillation induced by the stepper motor, as for example, conventional communications satellites.
2. Description of the Relevant Literature
The following references were noted during search of U.S. Patent Office records in respect to this invention:
European Patent Application No. 95,375 in the name of Rees. This application describes a spacecraft solar array pointing drive which uses a stepper motor, with the object of achieving smoother array movement with a small gear ratio and hence less of an overtorque requirement. The thrust of this disclosure is the achievement of fractional parts of the step through single step changes of the drive signals. However, the problem of mechanical resonance of the load is not addressed in this disclosure.
U.S. Pat. No. 3,828,234 to Goldberg. This patent is related to the generation of an error signal to drive a stepper motor. An error signal is generated as the difference between a reference signal and a phase signal derived from the windings the stepper motor. This patent is not particularly relevant to the subject invention.
U.S. Pat. No. 4,072,884 to Treadwell. This patent deals with speed and direction of rotation control of a brushless dc motor. This patent extends the teaching of the '234 patent above to include direction control. An error signal for driving the motor derived from a reference signal and a signal from the motor windings. The reference signal is variable.
U.S. Pat. No. 4,076,191 to Binge. This patent describes a mechanical arrangement of bearings and the preloading of bearings to provide high stiffness and to accommodate thermal expansion between shafts and housings. Stiffening may increase structural resonance frequencies in lateral directions but has no effect on resonant frequencies about the axis of rotation. Thus, this patent fails to offer any solution to the class of problem addressed by the present invention.